乒乓 is onomatopoeia for the sound of the ball bouncing back and forth. The characters literally depict a 丘 (mound) with a stroke knocked off - as if something got hit away.
Table tennis is China's 国球 ('national sport'). In the 1970s, 乒乓外交 ('ping-pong diplomacy') helped thaw US-China relations when the American table tennis team was invited to China in 1971, paving the way for Nixon's historic visit.
